1. Field of the Invention
This invention relates to a brushless motor used to drive a cylinder head in a rotary head type tape player, to drive an optical disk in an optical disk player or to drive a tape reel in a tape player and, more particularly, to a brushless motor which is stably rotated without irregularity in a gap between a stator coil and a rotor magnet.
2. Description of the Prior Art
FIG. 3 shows a half sectional view of a conventional brushless motor.
In a conventional brushless motor, a radial bearing 2 and a thrust bearing 3 for supporting a rotational shaft 1 are held in a case 4. A plurality of stator coils 6 are supported on a printed substrate 5 fixed to the case 4. A bracket 7 is fixed to the rotational shaft 1, a yoke 8 is fixed to the bracket 7, and a rotor magnet 9 is supported to the yoke 8. An opposite yoke 10 is fixed to the bracket 7, and the stator coil 6 is interposed between the yoke 10 and the rotor magnet 8.
In the brushless motor of this type, a current flows radially at the rotational shaft 1 as a center in the portion of the stator coil 6. The rotor magnet 9 is alternatively magnetized at N- and S-poles circumferentially. The rotational shaft 1 is rotatably driven by means of the electromagnetic force produced by the current flowing to the stator coil 6 and the pole of the rotor magnet 9. The rotary power is transmitted externally, for example, through a pulley 7a formed integrally with the bracket 7.
In the brushless motor of this type, it is preferable that a gap .delta. between the stator coil 6 and the rotor magnet 9 is constant in the entire opposed area of both. However, in the conventional structure shown in FIG. 3, the causes for determining the relatively mounting accuracy of the stator coil 6 and the rotor magnet 9 are induced by various factors, such as the overturn of the rotor magnet 9 with respect to the rotational shaft 1, the perpendicularity of the case 4 with respect to the radial bearing 2, the overturn of the printed substrate 5 with respect to the case 4. Thus, it is difficult to maintain the relative positional accuracy between the stator coil 6 and the rotor magnet 9 high, and the gap .delta. of both becomes irregular in the rotating range. When the gap .delta. becomes irregular in this manner, wow and flutter occur by the rotation of the rotor magnet 9, and the driving torque also becomes irregular, thus the rotation becoming unstable.